This invention relates to a hole filling method for a printed wiring board used in an electronic apparatus.
Referring to FIGS. 1A through 1G, description will be made of a related hole filling method for a printed wiring board using a printing technique.
Referring to FIG. 1A, preparation is made of a printed wiring board 1 to be subjected to a hole filling process. The printed wiring board 1 comprises an insulator substrate 1a and a copper plating layer 1b. The insulator substrate 1a has through holes formed therein as desired. Then, the insulator substrate 1a is subjected to copper plating to form the copper plating layer 1b on both surfaces of the insulator substrate 1a and inside surfaces of the through holes. The copper plating layer 1b formed on the inside surface of each through hole in the insulator substrate 1a has an inner surface defining an interlayer connection hole (simply referred to as an interlayer connection hole throughout the specification) of the printed wiring board 1. The interlayer connection holes include a large-diameter through hole 3 having a diameter not smaller than the thickness of the printed wiring board 1 and a small-diameter through hole 4 having a diameter smaller than the thickness of the printed wiring board 1.
The printed wiring board 1 prepared as mentioned above is an interlayer connection plating printed wiring board. The hole filling process is carried out for each single interlayer connection plating printed wiring board.
Referring to FIG. 1B, the printed wiring board 1 is placed on a printing table 40 in order to carry out the hole filling process using the printing technique. A printing plate or pattern 50 is used to print a hole filling resin (for example, epoxy resin) 9 at desired positions. A squeegee 70 serves to apply the hole filling resin 9. After the printed wiring board 1 is placed on the printing table 40, the printing pattern 50 is put on the printed wiring board 1 and the hole filling resin 9 is applied and spread on the printing pattern 50 covering the printed wiring board 1 by the use of the squeegee 70.
Referring to FIG. 1C, the hole filling resin 9 is applied and spread on the printing pattern 50 by the squeegee 70 to be filled in the through holes 3 and 4 as filled resins 19 and 20, respectively. After the through holes 3 and 4 are filled with the hole filling resin 9, the printing pattern 50 is removed and the printed wiring board 1 is displaced from the printing table 40.
Referring to FIG. 1D, the printing pattern 50 is removed from the printed wiring board 1 and the printed wiring board 1 is displaced from the printing table 40. In this state, the hole filling resin 9 is cured. In FIG. 1D, a resin residue 100 is formed by a part of the hole filling resin 9 which has been present in an opening portion of the printing pattern 50 upon filling the hole filling resin 9 in the through holes 3 and 4 and is left on a resin insertion surface (upper surface in the figure) of the printed wiring board 1 or which leaks from the periphery of the opening portion of the printing pattern 50. The resin residue 100 also includes a part of the hole filling resin 9 leaking from a gap between the printing pattern 50 and the printed wiring board 1 in the vicinity of open ends of the through holes 3 and 4. The hole filling resin 9 is cured by heating or photocuring. At this time, both of the filled resins 19 and 20 filled in the through holes 3 and 4 and the resin residue 100 are cured together. As a result, protrusions are formed on the surface of the printed wiring board 1. Such protrusions must be removed by polishing or the like.
Referring to FIG. 1E, the surface of the printed wiring board 1 is flattened and smoothed by the use of a polisher 110. In order to remove the protrusions formed on the surface of the printed wiring board 1 due to presence of the resin residue 100 when the hole filling resin 9 is cured and to flatten and smooth the surface of the printed wiring board 1, polishing is carried out by the use of the polisher 110. At this time, the surface of the printed wiring board 1 is also shaved.
Referring to FIG. 1F, the protrusions are removed by polishing to produce flattened surfaces 120 and 130 of the hole filling resin 9 filled in the through holes 3 and 4 as the filled resins 19 and 20, respectively. In this state, the through holes 3 and 4 may be referred to as filled through holes.
Referring to FIG. 1G, the printed wiring board 1 with the filled through holes is obtained through the above-mentioned steps. In the illustrated state, the through holes 3 and 4 are filled with the hole filling resin 9 as the filled resins 19 and 20, respectively.
Next referring to FIGS. 2 and 3, description will be made of a merit achieved by filling the hole filling resin in the through holes formed in the printed wiring board 1.
Referring to FIG. 2, a component 32 is mounted on the printed wiring board 1 in which the through holes are not filled. Referring to FIG. 3, the component 32 is mounted on the printed wiring board 1 in which the through holes are filled.
In FIG. 2, the hole filling resin 9 is not filled in the through hole 3 as the filled resin 19. Therefore, a connection pad to connect the component 32 can not be formed on the through hole 3. On the other hand, in FIG. 3, the through hole 3 is filled with the hole filling resin 9 as the filled resin 19. Therefore, a pad 34 comprising a copper plating layer or the like can be formed on the filled resin 19 filled in the through hole 3 in such a manner that the pad 34 is connected to the copper plating layer 1b of the printed wiring board 1. Then, the component 32 can be mounted on the pad 34 located on the through hole 3.
As illustrated in FIG. 3, the pad 34 is formed on the through hole 3 buried with the filled resin 19 and the component 32 is directly connected to the pad 34 located on the through hole 3. In this manner, the wiring distance is shortened and the inductance is reduced as compared with FIG. 2. Thus, the impedance of the printed wiring board 1 is reduced.
However, in the related hole filling method described in conjunction with FIG. 1, the hole filling process must be carried out for each single printed wiring board 1. Therefore, the productivity is inferior.
In addition, the resin filling step and the resin curing step must separately be carried out. Therefore, the productivity is inferior.
Furthermore, the hole filling process must be followed by polishing to improve the flatness of the surface of the printed wiring board. Therefore, the productivity is inferior.
It is an object of this invention to provide a hole filling method for a printed wiring board, which is improved in productivity.
It is a specific object of this invention to provide a hole filling method for a printed wiring board, which is capable of simultaneously carrying out a hole filling process for two printed wiring boards.
It is another specific object of this invention to provide a hole filling method for a printed wiring board, which is capable of simultaneously carrying out a resin filling step and a resin curing step.
It is still another specific object of this invention to provide a hole filling method for a printed wiring board, which is capable of improving the flatness of the surface of the printed wiring board without requiring a polishing step after a hole filling process.
According to this invention, there is provided a hole filling method for a printed wiring board having an interlayer connection hole, comprising the steps of:
preparing first and second printed wiring boards each of which has the interlayer connection hole;
placing the first printed wiring board on a bottom plate;
placing on the first printed wiring board a first hole guide plate having a first guide hole formed at a position corresponding to the interlayer connection hole of the first printed wiring board in such a manner that the first guide hole is coincident with the interlayer connection hole of the first printed wiring board;
superposing a hole filling resin on the first hole guide plate;
placing on the hole filling resin a second hole guide plate having a second guide hole formed at a position corresponding to the interlayer connection hole of the second printed wiring board;
placing on the second hole guide plate the second printed wiring board in such a manner that the interlayer connection hole of the second printed wiring board is coincident with the second guide hole of the second hole guide plate;
placing a top plate on the second printed wiring board to form a stacked assembly which comprises the first printed wiring board, the first hole guide plate, the hole filling resin, the second hole guide plate, and the second printed wiring board between the bottom plate and the top plate; and
vacuum-pressing the stacked assembly between the bottom plate and the top plate to simultaneously fill the interlayer connection holes of the first and the second printed wiring boards with a part of the hole filling resin.
Typically, each of the first and the second printed wiring boards comprises: an insulator substrate having a through hole (of any shape); and a metal film formed on a surface of the insulator substrate and on an inside surface of the through hole. The interlayer connection hole of each of the first and the second printed wiring boards is defined by an inner surface of the metal film formed on the inside surface of the through hole in each of the first and the second printed wiring boards.
Preferably, a thermosetting resin is used as the hole filling resin. The vacuum-pressing step is for vacuum-pressing the stacked assembly between the bottom plate and the top plate with the stacked assembly heated.
Thus, this invention is applicable to the printed wiring board on which a component is mounted on the interlayer connection hole after it is filled. A parting plate is placed as the bottom plate. On the parting plate, the printed wiring board with the interlayer connection hole not yet filled is placed. On the printed wiring board, the hole guide plate as a guide is placed. The hole filling resin is superposed on the hole guide plate. On the hole filling resin, the hole guide plate, the printed wiring board, and the parting plate as the top plate are similarly superposed to be symmetrical with those under the hole filling resin. The stacked assembly including the two printed wiring boards placed above and below are collectively subjected to heating and vacuum pressing. Thus, this invention provides the hole filling method using pressing with the hole guide plates, which is capable of carrying out the hole filling process simultaneously for the two printed wiring boards, which is capable of flattening a fracture surface of the hole filling resin after the hole filling process without forming the protrusions on the printed wiring board because the hole guide plate has a sealing effect of preventing leakage of the hole filling resin around the interlayer connection hole of the printed wiring board to be subjected to the hole filling process and the hole guide plate provides a function as a resin insertion guide hole, and which is free from limitation in relationship between the diameter of the interlayer connection hole to be filled and the thickness of the printed wiring board.